This invention relates a method for preparing superconductive Nb.sub.3 Sn layers on niobium surfaces for high frequency applications in general and more particularly to an improved method of this type.
Superconducting apparatus for operation with high frequency electromagnetic fields at frequencies of about 10 MHz and higher can have many technical applications. In particular, such apparatus can be used as resonators and separators for particle accelerators or as high frequency resonators for other purposes, for instance, as frequency standards. For this purpose they can be designed as cavity resonators or resonator helices. Superconducting cavity resonators are operated in the frequency range of about 1 to 15 GHz and resonator helices in the range around 100 MHz. Primary niobium and occasionally also lead have heretofore been used as superconductor materials for such resonators.
In such superconducting apparatus, it is desired to have a large Q factor and also, as a rule, the highest possible critical magnetic flux density B.sub.c.sup.ac as measured under the influence of high frequency fields, in order to operate the superconducting apparatus with as little high frequency power as possible and, at the same time, have a low surface resistance. For, if the critical magnetic flux density B.sub.c.sup.ac is exceeded, the losses increase steeply, the surface resistance increases considerably and the electromagnetic field breaks down. An upper limit for B.sub.c.sup.ac is given in such cases by what is known as the thermodynamic critical flux density B.sub.c. Since the critical thermodynamic flux density B.sub.c of Nb.sub.3 Sn is higher than that of niobium, it stands to reason that a higher critical flux density B.sub.c.sup.ac can be achieved at an Nb.sub.3 Sn surface than at the niobium surface. In addition, Nb.sub.3 Sn also has a considerably higher critical temperature than niobium, so that it has, on the one hand, greater thermal stability and, on the other hand, should also permit higher operating temperatures than niobium, particularly for operation at the temperature of boiling liquid helium of 4.2 K, which is already too high for high frequency applications of niobium.
There have been previous attempts at applying thin protective layers of Nb.sub.3 Sn on niobium resonators by first evaporating tin on the niobium resonator and then heat treating the resonator. With such surface layers, a Q.sub.o of about 10.sup.9 and a critical flux density B.sub.c.sup.ac of about 25 mT were measured at 2.8 GHz (cf. "Siemens-Forschungs-und Entwicklungsberichte" 3 (1974), pages 90 to 99).
With such a procedure, however, a difficulty is encountered in that the evaporated tin melts at the beginning of the heat treatment and can run, for instance, where the interior of a cavity resonator is being coated, along the interior surface to the lowest point of the cavity, before enough tin for the formation of a sufficiently thick Nb.sub.3 Sn layer has been diffused into the niobium surface. Therefore, in practice only very thin tin layers can be vapor deposited and the evaporation and subsequent heat treatment must be repeated several times before a sufficient amount of tin for forming the Nb.sub.3 Sn layer can diffuse into the niobium surface.
It has also been proposed to expose the niobium parts which are to be provided with an Nb.sub.3 Sn layer to a tin vapor atmosphere, from which the tin diffuses into the niobium surface, forming the desired Nb.sub.3 Sn layer, in a closed reaction vessel, i.e., a sealed quartz ampoule, at an elevated temperature of about 1000.degree. C. Thereby, Nb.sub.3 Sn layers of a thickness of several micrometers with relatively good properties, e.g., Q.sub.o values of about 10.sup.9 and critical magnetic flux densities B.sub.c.sup.ac of somewhat over 40 mT, can be obtained (See the paper by Hillenbrand et al in "IEEE Transactions of Magnetics," vol. MAG-11, No. 2, Mar. 1975, Pages 420 to 422).